The search for compositions which have a combination of excellent insecticidal activity and desirable low toxicity to plants and mammals is a continuing one because of factors such as the desire for compounds exhibiting greater insecticidal activity, better selectivity, low environmental impact, low production cost and effectiveness against insects resistant to many known insecticides.
Various parts of the neem (or nim) tree have long been used in India for their reputed medicinal or insecticidal properties. This subtropical tree is native to the arid regions of India, Pakistan, Sri Lanka and parts of Southeast Asia and Africa.
Although all parts of the neem tree appear to have natural resistance to pests and diseases, the seeds appear to have the greatest resistance. Formulations and extracts of the seeds have been shown to be effective against many species of crop pests including gypsy moths, Japanese beetles, aphids, tobacco budworms and boll weevils. For example, see Chem. and Engineering News, May 27, 1985, pp. 46-51 and U.S. Dep. Agric., Agric. Rev. Man., ARM-NE-4. Neem seed extract is considered to be a broad-spectrum insecticide.
Azadirachtin and other insecticidally active neem seed components can be extracted from ground neem seeds using polar solvents such as water, methanol or ethanol. On a dry weight basis, such extracts contain typically less than 5% of azadirachtin. Due to decomposition of the active ingredients, the potency of such extracts as insecticides rapidly decreases. Thus, the utility of such neem seed extracts as useful plant protection materials is severely limited.
As disclosed in U.S. Pat. No. 4,556,562, longer shelf stability is possible to achieve by formulating crude extracts as dilute alcoholic solutions, typically containing about 0.3% of azadirachtin, and adjusting the pH of those extracts to about 3.5-6. Alternatively, as disclosed in U.S. Pat. No. 4,946,681, a method is described for stabilizing an alcoholic neem seed extract by removing water with molecular sieves.
There are several known procedures for obtaining extracts of high purity, typically containing 10-100% of azadirachtin. The common feature of all these procedures is the use of polar organic solvents, such as methanol, to obtain crude extracts from which azadirachtin is separated in a succession of process steps involving liquid-liquid extractions, phase separations, solvent strips and sometimes chromatography on silica gels. Typically, the most hydrophobic impurities such as triglycerides are removed by extracting dry crude extracts or crude extracts dissolved in 50-100% aqueous methanol with a non-polar solvent such as hexane (Journal of Liquid Chromatography, 10 (6), 1151 (1987)). Following this procedure, the extract is partitioned between water and a polar organic solvent which is immiscible with water, such as ethyl acetate, and the organic solvent phase stripped to yield a semi-pure extract, typically containing less than 15 to 25% of azadirachtin, having a limited storage stability. Chromatography on silica gel, florasil and the like is typically required in order to obtain higher purity compositions of azadirachtin.
This known methodology is impractical and costly when it is scaled-up because it requires the handling of large volumes of flammable solvents relative to the quantity of azadirachtin that is recovered. The problems are aggravated by the fact that polar organic solvents such as methanol extract a great many accompanying impurities which subsequently need to be separated in order to prevent rapid decomposition of azadirachtin in such extracts.
Replacement of the alcohol extraction solvent with water would significantly reduce the extractability of many hydrophobic impurities and, in addition, would eliminate the flammability issue. However, despite the knowledge of the ability of water to extract azadirachtin from ground neem seeds, there are no published methods which take advantage of this fact. The reason for this is that separation of a clear aqueous extract from the extracted neem cake is very slow if it is done by commonly used filtration methods. Additionally, all attempts to extract azadirachtin from the aqueous filtrate with a polar organic solvent such as ethyl acetate fail due to the formation of stable emulsions which are difficult to break down.
Therefore, there remained a need for a practical, scalable and economical method that yields a solvent free composition in which azadirachtin is present in over a 15% concentration. Such a composition should possess a sufficient stability that would allow it to be used in a wide variety of insecticidal applications or, if necessary, would be amenable to further stabilization by simple chemical methods.